Process for the production of a two-coat finish, and nonaqueous coatings suitable for this process

ABSTRACT

The invention relates to a process for the production of two-coat finishes of the basecoat/clearcoat type, in which nonaqueous transparent topcoats are employed, which comprise 
     (A) a synthetic resin containing hydroxyl groups or a mixture of synthetic resins containing hydroxyl groups, 
     (B) an amino resin or a mixture of amino resins, and 
     (C) a blocked polyisocyanate or a mixture of blocked polyisocyanates, 
     in which component (C) comprises isocyanate groups blocked both with a blocking agent (I) and with a blocking agent (II), and the blocking agent (I) being a dialkyl malonate, the blocking agent (II) being a blocking agent which is different from (I) and contains active methylene groups or an oxime, and the ratio of equivalents between the isocyanate groups blocked with (I) and the isocyanate groups blocked with (II) being between 1.0:1.0 and 9.0:1.0.

The invention relates to a process for the production of a two-coatfinish, in which

(1) a pigmented basecoat is applied to the substrate surface,

(2) a polymer film is formed from the basecoat applied in step (1),

(3) a nonaqueous transparent topcoat is applied to the resultingbasecoat layer, and subsequently

(4) basecoat layer and topcoat layer are baked together.

The invention also relates to nonaqueous coatings suitable for thisprocess.

The basecoat/clearcoat process described above is known and is employedin particular for producing finishes, in particular metallic finishes oncar bodies (c.f. e.g. U.S. Pat. No. 3,639,147 and EP-A-38 127).

Finishes which can be prepared using the basecoat/clearcoat process, incomparison to single-coat finishes, give an improved effect and enablethe production of finishes with bright, purer colors.

The basecoat initially applied in step (1) determines, depending on thetype, amount and spatial orientation of the pigments employed, the colorand, if appropriate, the effect (e.g. metallic effect or pearlescenteffect) of the finish.

In step (2) of the process, at least part of the organic solvents and/orat least part of the water is removed in a flash-off phase from thebasecoat film applied in step (1). In step (3) a nonaqueous transparenttopcoat is applied to this predried but unbaked basecoat layer(wet-on-wet method) and then in step (4) the basecoat layer and topcoatlayer are baked together.

The transparent topcoat applied in step (3) gives the two-coat finishgloss and fullness, and protects the pigmented paint layer applied instep (1) against chemical and physical attack.

High-quality two-coat finishes can only be obtained by the process underdiscussion if the transparent topcoat applied in step (3) does notinterfere with the basecoat layer applied in steps (1) and (2) in such away that the optical effect is impaired (e.g. clouding). On the otherhand, the transparent topcoat must have a composition such that afterthe baking process carried out in step (4) it adheres well to thebasecoat layer. Further important characteristics which the transparenttopcoat layer obtained after the baking process must have are hightransparency, high gloss and good mechanical properties, such ashardness, scratch resistance and elasticity. The transparent topcoatlayer obtained after the baking process must have not least a highdegree of resistance to climatic effects (e.g. temperature fluctuations,moisture in the form of water vapor, rain, dew, radiation stress etc)and to attack by acids or other chemicals, such as organic solvents. Inaddition, the transparent topcoats applied in step (3) should have aslow a content of organic solvents as possible and good stability onstorage.

JP-A-2-242,867 describes a basecoat/clearcoat process in which, in step(3), nonaqueous transparent topcoats are applied, which comprise (A) asynthetic resin containing hydroxyl groups, (B) an amino resin and (C) ablocked polyisocyanate, where components (B) and (C) are to be chosensuch that the temperature at which a chemical reaction occurs between(A) and (C) should be not more than 20° C. below and not more than 50°C. above the temperature at which a chemical reaction between (A) and(B) starts.

Blocking agents which are mentioned for the preparation of component (C)are: volatile, low molecular weight compounds containing active hydrogenatoms, such as methanol, ethanol, propanol, butanol, hexanol,cyclohexanol, benzyl alcohol, ethylene glycol monoethyl ether and otheraliphatic or aromatic monoalcohols, dimethyl- or diethyl-aminoethanoland other tertiary amines containing hydroxyl groups, acetone oxime,methyl ethyl ketone oxime and other oximes, acetylacetone,acetoacetates, malonates and other compounds containing active methylenegroups, ε-caprolactam and other lactams, and phenol. The blocking agentspreferably employed are aliphatic monoalcohols, oximes and caprolactams.

The transparent topcoats described in JP-A-2-242,867 give finishes whichrequire improvement, in particular with regard to their resistance toorganic solvents and acids, their gloss, transparency and resistance toyellowing.

DE-B-2,639,491 describes nonaqueous coatings which comprise a polyesterresin and/or alkyd resin containing hydroxyl groups, hexamethylenediisocyanate blocked with an alkyl acetoacetate and/or2,2,4-trimethylhexamethylene diisocyanate blocked with an alkylacetoacetate, and an amino resin. These coatings can also be employed astransparent topcoats in the automotive finishing sector. Finishesobtained using these coatings undergo yellowing, especially when usinghigh baking temperatures and/or prolonged baking times, and requireimprovement, in particular with regard to their resistance to acids andorganic solvents and to their scratch resistance.

The subject of the present invention is a process of the type mentionedat the beginning in which, in step (3), a nonaqueous transparent topcoatis applied, which comprises

(A) a synthetic resin containing hydroxyl groups or a mixture ofsynthetic resins containing hydroxyl groups,

(B) an amino resin or a mixture of amino resins, and

(C) a blocked polyisocyanate or a mixture of blocked polyisocyanates,

in which component (C) contains isocyanate groups blocked both with ablocking agent (I) and with a blocking agent (II),

the blocking agent (I) being a dialkyl malonate or a mixture of dialkylmalonates,

the blocking agent (II) being a blocking agent which is different from(I) and contains active methylene groups, an oxime or a mixture of theseblocking agents, and

the ratio of equivalents between the isocyanate groups blocked with (I)and the isocyanate groups blocked with (II) being between 1.0:1.0 and9.0:1.0.

The two-coat finishes produced by the process according to the inventionare notable for a high level of hardness, high gloss, good adhesionbetween the basecoat layer and topcoat layer, good topcoat appearance,good scratch resistance and good resistance to climatic effects, organicsolvents and acids, and a high resistance to yellowing (in particular toyellowing resulting from high baking temperatures and/or long bakingtimes). These favorable properties are retained even if differentbasecoats are used. The transparent topcoats employed according to theinvention are also notable for high stability on storage and can also beprocessed well with a low content (e.g. less than 50% by weight) oforganic solvents.

EP-A-403,044 describes coatings containing blocked polyisocyanates, inwhich each polyisocyanate molecule is blocked with at least twodifferent blocking agents and at least two of the blocking agents have aconsiderable difference, preferably at least 40° C., in their deblockingtemperatures. The coatings described in EP-A-403,044 do not contain anamino resin and are employed for coil-coating applications. EP-A-403,044contains no mention of the basecoat/clearcoat process on which thepresent invention is based.

In step (I) of the process according to the invention, it is inprinciple possible to employ all pigmented basecoats suitable for thepreparation of two-coat finishes. Such basecoats are well known to thoseskilled in the art. It is possible to employ both water-thinnablebasecoats and basecoats based on organic solvents. Suitable basecoatsare described in, for example, U.S. Pat. No. 3,639,147, DE-A-3,333,072,DE-A-3,814,853, GB-A-2,012,191, U.S. Pat. No. 3,953,644, EP-A-260,447,DE-A-3,903,804, EP-A-320,552, DE-A-3,628,124, U.S. Pat. No. 4,719,132,EP-A-297,576, EP-A-69,936, EP-A-89,497, EP-A-195,931, EP-A-228,003,EP-A-38,127 and DE-A-2,818,100. These patent documents also containfurther information on the basecoat/clearcoat process under discussion.

In step (2) of the process according to the invention the solventsand/or the water are removed in a flash-off phase from the basecoatapplied in step (1). The basecoat layer can also be baked. However, thisis disadvantageous on economic grounds because then two bakingoperations instead of one are required to produce the two-coat finish.

The nonaqueous transparent topcoat employed in step (3) of the processaccording to the invention comprises

(A) a synthetic resin containing hydroxyl groups or a mixture ofsynthetic resins containing hydroxyl groups,

(B) an amino resin or a mixture of amino resins, and

(C) a blocked polyisocyanate or a mixture of blocked polyisocyanates,

in which component (C) is blocked both with a blocking agent (I) andwith a blocking agent (II),

the blocking agent (I) being a dialkyl malonate or a mixture of dialkylmalonates,

the blocking agent (II) being a blocking agent which is different from(I) and contains active methylene groups, an oxime or a mixture of theseblocking agents, and

the ratio of equivalents between the isocyanate groups blocked with (I)and the isocyanate groups blocked with (II) being between 1.0:1.0 and9.0:1.0.

The component (A) employed can in principle be any synthetic resincontaining hydroxyl groups which is suitable for transparent topcoats,or a mixture of such synthetic resins. As component (A) it is preferredto employ polyester resins containing hydroxyl groups and/or alkydresins containing hydroxyl groups and/or polyacrylate resins containinghydroxyl groups, or mixtures of these resins. The synthetic resinsemployed as component (A) in general have hydroxyl numbers of from 40 to240, preferably 60 to 150, and number-average molecular weights of from1500 to 30,000, preferably 2000 to 15,000, and particularly preferably2500 to 7500.

Polyester resins, alkyd resins and polyacrylate resins containinghydroxyl groups are well known. Examples of such resins and theirpreparation are described in, for example, JP-A-2-242,867,DE-B-2,639,491 and in the patent documents mentioned on page 6 in lines20 to 25.

As component (A) it is particularly preferred to employ polyacrylateresins which can be prepared by polymerizing

(a) from 10 to 92, preferably 20 to 60, % by weight of an alkyl orcycloalkyl acrylate or of an alkyl or cycloalkyl methacrylate having 1to 18, preferably 4 to 13, carbon atoms in the alkyl or cycloalkylmoiety, or mixtures of such monomers,

(b) from 8 to 60, preferably 12.5 to 38.5, % by weight of a hydroxyalkylacrylate or of a hydroxyalkyl methacrylate having 2 to 4 carbon atoms inthe hydroxyalkyl moiety, or mixtures of such monomers,

(c) from 0.0 to 5.0, preferably 0.7 to 3.0, % by weight of acrylic acidor methacrylic acid, or mixtures of these monomers, and

(d) from 0 to 50, preferably 0 to 30, % by weight of ethylenicallyunsaturated monomers which are different from (a), (b) and (c) and canbe copolymerized with (a), (b) and (c), or mixtures of such monomers

to give polyacrylate resins having hydroxyl numbers of from 40 to 240,preferably 60 to 150, acid values of from 0 to 35, preferably from 5 to20, glass transition temperatures of from -35° to +70° C., preferablyfrom -20° to +40° C., and number-average molecular weights of from 1500to 30,000, preferably from 2000 to 15,000 (determined by gel permeationchromatography using a polystyrene standard).

Examples of (a)-components are: methyl, ethyl, propyl, n-butyl,isobutyl, tert-butyl, pentyl, hexyl, heptyl and 2-ethylhexyl acrylateand methacrylate, and cyclohexyl acrylate and cyclohexyl methacrylate.

Examples of (b)-components are: hydroxyethyl, hydroxypropyl andhydroxybutyl acrylate and methacrylate.

Examples of (d)-components are: aromatic vinyl compounds, for examplestyrene, vinyltoluene, α-methylstyrene, α-ethylstyrene, ring-substituteddiethylstyrene, isopropylstyrene, butylstyrenes and methoxystyrenes;vinyl ethers, for example ethyl vinyl ether, n-propyl vinyl ether,isopropyl vinyl ether, n-butyl vinyl ether and isobutyl vinyl ether, andvinyl esters, for example vinyl acetate, vinyl propionate, vinylbutyrate, vinyl pivalate and the vinyl ester of2-methyl-2-ethylheptanoic acid.

The hydroxyl number and the acid value of the polyacrylate resins caneasily be controlled by those skilled in the art via the amount ofcomponent (b) and (c) employed.

The glass transition temperature of the polyacrylate resins isdetermined by the type and amount of the monomers employed. The monomerscan be selected by those skilled in the art with the aid of thefollowing formula, which can be used to calculate approximately theglass transition temperatures of polyacrylate resins: ##EQU1## T_(G)=glass transition temperature of the polyacrylate resin x=number ofdifferent monomers copolymerized in the polyacrylate resin.

W_(n) =proportion by weight of the nth monomer

T_(Gn) =glass transition temperature of the homopolymer of the nthmonomer.

Measures to control the molecular weight (e.g. the selection ofappropriate polymerization initiators, the use of chain transfer agentsetc.) are part of the expert knowledge of those of average skill in theart, and require no more detailed discussion here.

As component (A), it is also particularly preferred to employ polyesterresins or alkyd resins which can be prepared by reacting

(α) a cycloaliphatic or aliphatic polycarboxylic acid or a mixture ofsuch polycarboxylic acids,

(β) an aliphatic or cycloaliphatic polyol having more than two hydroxylgroups in the molecule, or a mixture of such polyols,

(γ) an aliphatic or cycloaliphatic diol or a mixture of such diols, and

(δ) an aliphatic, linear or branched, saturated monocarboxylic acid or amixture of such monocarboxylic acids

in a molar ratio of (α):(β):(γ):(δ)=1.0:0.2-1.3:0.0-1.1:0.0-1.4,preferably 1.0:0.5-1.2:0.0-0.6:0.2-0.9 to give a polyester resin oralkyd resin.

Examples of component (α) are: hexahydrophthalic acid,1,4-cyclohexanedicarboxylic acid, endomethylenetetrahydrophthalic acid,oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid,pimelic acid, suberic acid, azelaic acid and sebacic acid.

Examples of component (β) are: pentaerythritol, trimethylolpropane,trimethylolethane and glycerol.

Examples of component (γ) are: ethylene glycol, diethylene glycol,propylene glycol, neopentyl glycol, 2-methyl-2-propylpropane-1,3-diol,2-ethyl-2-butyl-propane-1,3-diol, 2,2,4-trimethylpentane-1,5-diol,2,2,5-trimethylhexane-1,6-diol, neopentyl glycol hydroxypivalate anddimethylolcyclohexane.

Examples of component (δ) are: 2-ethylhexanoic acid, lauric acid,isooctanoic acid, isononanoic acid and mixtures of monocarboxylic acidswhich are obtained from coconut oil or palm kernel oil.

The preparation of polyester resins and/or alkyd resins carryinghydroxyl groups is described in, for example, Ullmanns Encyklopadie dertechnischen Chemie, third edition, volume 14, Urban & Schwarzenberg,Munich, Berlin 1863, pages 80 to 89 and pages 99 to 105, and in thebooks: Resines Alkydes-Polyesters by J. Bourry, Paris Verlag Dunod 1952,Alkyd resins by C. R. Martens, Reinhold Publishing Corporation, New York1961 and Alkyd Resin Technology by T. C. Patton, Interscience Publishers1962.

As component (B), in principle any amino resin suitable for transparenttopcoats, or a mixture of such amino resins, can be employed.

Such resins are well known to those skilled in the art and are marketedby numerous companies as commercial products. Amino resins arecondensation products of aldehydes, especially formaldehyde, and, forexample, urea, melamine, guanamine and benzoguanamine. The amino resinscontain alcohol groups, preferably methylol groups, of which some, orpreferably all, are etherified with alcohols.

As component (B), it is preferred to employ melamine-formaldehyde resinsetherified with lower alcohols, in particular with methanol or butanol.It is particularly preferred to employ melamine-formaldehyde resinsetherified with lower alcohols, in particular with methanol and/orbutanol, which on statistical average still contain 0.1 to 0.25 hydrogenatoms attached to nitrogen atoms per triazine ring, as component (B).

The transparent topcoats employed according to the invention contain ascomponent (C) a blocked polyisocyanate or a mixture of blockedpolyisocyanates in which component (C) contains isocyanate groupsblocked both with a blocking agent (I) and with a blocking agent (II),

the blocking agent (I) being a dialkyl malonate or a mixture of dialkylmalonates,

the blocking agent (II) being a blocking agent which is different from(I) and contains active methylene groups, an oxime or a mixture of theseblocking agents, and

the ratio of equivalents between the isocyanate groups blocked with (I)and the isocyanate groups blocked with (II) being between 1.0:1.0 and9.0:1.0, preferably between 8.0:2.0 and 6.0:4.0, particularly preferablybetween 7.5:2.5 and 6.5:3.5.

Component (C) is preferably prepared as follows. A polyisocyanate or amixture of polyisocyanates is reacted in a manner known per se with amixture of the blocking agents (I) and (II), the mixture of the blockingagents (I) and (II) containing the blocking agents (I) and (II) in amolar ratio which is between 1.0:1.0 and 9.0:1.0, preferably between8.0:2.0 and 6.0:4.0, particularly preferably between 7.5:2.5 and6.5:3.5. The polyisocyanate or the mixture of polyisocyanates can bereacted with the mixture of the blocking agents (I) and (II) to thepoint where no more isocyanate groups can be detected. In practice thismay require the use of very large excess amounts of blocking agentsand/or very long reaction times. It has now been found that,surprisingly, coatings with the good properties described above are alsoobtained if at least 50 percent, preferably at least 70 percent, of theisocyanate groups of the polyisocyanate or of the mixture ofpolyisocyanates are reacted with the mixture of the blocking agents (I)and (II), and the remaining isocyanate groups are reacted with acompound containing hydroxyl groups or with a mixture of compoundscontaining hydroxyl groups. The compounds containing hydroxyl groupswhich are employed are preferably low molecular weight aliphatic orcycloaliphatic polyols such as neopentyl glycol, dimethylolcyclohexane,ethylene glycol, diethylene glycol, propylene glycol,2-methyl-2-propylpropane-1,3-diol, 2-ethyl-2-butylpropane-1,3-diol,2,2,4-trimethylpentane-1,5-diol and 2,2,5-trimethylhexane-1,6-diol, orthe synthetic resins containing hydroxyl groups which can be employed ascomponent (A).

Component (C) can also be obtained by mixing polyisocyanates blockedwith the blocking agents [sic] (I) or (II) in a ratio such that amixture is obtained in which the ratio of equivalents between theisocyanate groups blocked with (I) and the isocyanate groups blockedwith (II) is between 1.0:1.0 and 9.0:1.0, preferably between 8.0:2.0 and6.0:4.0, particularly preferably between 7.5:2.5 and 6.5:3.5. Thisprocedure for preparing component (C) is less preferred.

It is in principle possible to employ all polyisocyanates which can beemployed in the coating sector for the preparation of component (C).However, it is preferred to employ polyisocyanates whose isocyanategroups are attached to aliphatic or cycloaliphatic radicals. Examples ofsuch polyisocyanates are hexamethylene diisocyanate, isophoronediisocyanate, trimethylhexamethylene diisocyanate, dicyclohexylmethanediisocyanate and 1,3-bis(2-isocyanato-2-propyl)benzene (TMXDI) andadducts of these polyisocyanates with polyols, in particular lowmolecular weight polyols, for example trimethylolpropane, andpolyisocyanates which are derived from these polyisocyanates and containisocyanurate groups and/or biuret groups. As polyisocyanates it isparticularly preferred to employ hexamethylene diisocyanate andisophorone diisocyanate, polyisocyanates which are derived from thesediisocyanates, contain isocyanurate or biuret groups and whichpreferably contain more than two isocyanate groups per molecule, andreaction products of hexamethylene diisocyanate and isophoronediisocyanate, or a mixture of hexamethylene diisocyanate and isophoronediisocyanate, containing 0.3-0.5 equivalents of a low molecular weightpolyol having a molecular weight of from 62 to 500, preferably from 104to 204, in particular a triol, for example trimethylolpropane.

As blocking agent (I), dialkyl malonates or a mixture of dialkylmalonates are employed.

Examples of dialkyl malonates which can be employed are dialkylmalonates having 1 to 6 carbon atoms in each of the alkyl radicals, forexample dimethyl malonate and diethyl malonate, with diethyl malonatebeing preferably employed.

As blocking agents (II), blocking agents which are different from (I)and contain active methylene groups, and oximes and mixtures of theseblocking agents are employed. Examples of blocking agents which can beemployed as blocking agent (II) are: methyl, ethyl, propyl, butyl,pentyl, hexyl, heptyl, octyl, nonyl, decyl or dodecyl acetoacetate,acetone oxime, methyl ethyl ketoxime, acetylacetone, formaldoxime,acetaldoxime, benzophenoxime [sic], acetoxime and diisobutyl ketoxime.As blocking agent (II) it is preferred to employ an alkyl acetoacetatehaving 1 to 6 carbon atoms in the alkyl moiety or a mixture of suchalkyl acetoacetates or a ketoxime or a mixture of ketoximes. Ethylacetoacetate or methyl ethyl ketoxime is particularly preferablyemployed as blocking agent (II).

Components (A), (B) and (C) are generally employed in the transparenttopcoats employed according to the invention in amounts such thatcomponent (A) is present in an amount of from 50 to 90, preferably 60 to75, % by weight, component (B) in an amount of from 5 to 45, preferably10 to 25, % by weight and component (C) in an amount of from 5 to 45,preferably 10 to 25, % by weight, the percentages by weight being basedon (A)+(B)+(C)=100% by weight.

The transparent topcoats employed according to the invention contain nopigments or only transparent pigments. As organic solvents, the topcoatscontain customary organic solvents conventionally used in the productionof coatings. In addition, the topcoats can also contain furthercustomary additives, for example light stabilizers, leveling assistantsetc.

The two-coat finishes produced with the topcoats employed according tothe invention have the advantageous characteristics in particular evenwhen they have been baked under the baking conditions currently employedin automotive production-line finishing (30 minutes at 130° C. or 20minutes at 140° C.).

The invention is illustrated in more detail in the following examples.Amounts and percentages are to be understood as being by weight, unlessexpressly stated otherwise.

PREPARATION OF COMPONENT (A) EXAMPLE 1

Preparation of a Polyacrylate Resin Containing Hydroxyl Groups (AcrylateResin 1)

1140 g of polymerization solvent comprising a fraction of aromatichydrocarbons having a boiling range from 158° C. to 172° C. atatmospheric pressure are weighed out and charged to a jacketed stainlesssteel laboratory reactor with a useful capacity of 4 l, and with abottom valve, equipped with a heating system which can be controlled byan oil circulation thermostat, a paddle stirrer with stirrer seal,driven by an electric motor having a power consumption of 230 W, athermometer for monitoring the temperature of the reaction mixture, aninlet pipe for a stream of protective nitrogen gas, one metering vesseleach for a monomer mixture and an initiator solution, and a refluxcondenser. The solvent is heated to 150° C. Then, simultaneously, amixture of 562 g of tert-butyl acrylate, 182 g of n-butyl methacrylate,364 g of 2-hydroxypropyl methacrylate, 73 g of 4-hydroxybutyl acrylateand 33 g of acrylic acid is metered in from the metering vessel for themonomer mixture, and a solution of 73 g of tert-butyl perbenzoate in 73g of the aromatic solvent described above is metered in from themetering vessel for the initiator solution. The monomer mixture and theinitiator solution are metered evenly into the reactor over a period of4 hours and over a period of 4.5 hours respectively. The temperature isheld at 150° C. during this time. When the addition of the initiator iscomplete, the reaction mixture is held at 150° C. for a further hour andthen the degree of conversion is determined by multiple determination ofthe nonvolatile fraction of the reaction mixture (15 min in acirculating-air oven at 180° C.). When conversion is complete, 526 g ofthe polymerization solvent are distilled off under vacuum at 150 to 190hPa and 110° C. The batch is then diluted with 101 g of 1-methoxypropyl2-acetate and adjusted with the abovementioned aromatic solvent to anonvolatile fraction of approximately 60% by weight. The resultingpolymer solution has a nonvolatile fraction (measured in acirculating-air oven; 60 min at 130° C.) of 59.5%. The polymer has anacid value of 23.6 and an OH-number of 139, and a solution viscosity of390 mPa.s, as measured in the solution described in an ICI plate-coneviscometer at 23° C.

EXAMPLE 2

Preparation of a Polyacrylate Resin Containing Hydroxyl Groups (AcrylateResin 2)

The procedure is as in Example 1. 1033 g of the aromatic solventdescribed in Example 1 are weighed out and charged to the apparatusdescribed in that example, and heated to 140° C. A mixture of 131 g of acommercially available mixture of esters comprising the isomers oftridecyl alcohol with methacrylic acid, 269 g of n-butyl acrylate, 197 gof n-butyl methacrylate, 393 g of styrene, 131 g of 2-hydroxyethylmethacrylate, 157 g of 2-hydroxypropyl methacrylate and 33 g of acrylicacid is weighed out and transferred to the metering vessel for themonomers. A solution of 79 g of di-tert-butyl peroxide in 79 g of thearomatic solvent mentioned is weighed out and transferred to themetering vessel for the initiator solution. The contents of the meteringvessel for monomers and the contents of the metering vessel for theinitiator solution are metered uniformly into the polymerization solventover 4 hours and 4.75 hours respectively. The metering of the initiatorsolution is started 15 minutes before the metering of the monomermixture. The temperature is held at 140° C. during this time. When theaddition of the initiator is complete, the reaction mixture is held at140° C. for a further hour, and then the degree of conversion isdetermined by multiple determination of the nonvolatile fraction of thereaction mixture (15 min in a circulating-air oven at 180° C.). Whenconversion is complete, the batch is cooled and adjusted to anonvolatile fraction of approximately 55% with the aromatic solventdescribed. The resulting polymer solution has a nonvolatile fraction(measured in a circulating-air oven; 60 min at 130° C.) of 55.4%. Thepolymer has an acid value of 22.8 and an OH-number of 89, and a solutionviscosity of 660 mPa.s, as measured in the solution described in an ICIplate-cone viscometer at 23° C.

EXAMPLE 3

Preparation of an Alkyd Resin Containing Hydroxyl Groups

1142 g of hexahydrophthalic anhydride, 1024 g of1,1,1-trimethylolpropane, 527 g of isononanoic acid, as an isomermixture of 3,3,5-trimethylhexanoic acid and 3,5,5 trimethylhexanoicacid, and 100 g of xylene as an entrainer are weighed out and charged tothe apparatus described in Example 1, but with the metering vessels andreflux condenser replaced by a water separator and reflux condenser. Thewater separator is filled with xylene. The contents of the apparatus areheated over 8 hours to 210° C. so that there is a steady reflux of theentraining agent. The reaction mixture is held at 210° C. until an acidvalue of 18.6 and a viscosity of 940 mPa.s, measured on a sample of a60% solution of the reaction mixture in the aromatic solvent describedin Example 1, are reached. The contents of the apparatus are then cooledto 160° C. and dissolved with stirring in 1000 g of the abovementionedaromatic solvent, and the solution is then run off from the apparatus.The solution is then diluted with a quantity of the aromatic solventsuch that a nonvolatile fraction of 60.5% (measured in a circulating-airoven; 60 min at 130° C.) results. The alkyd resin prepared in this wayhas an acid value of 17.1, an OH-number of 123 based on the nonvolatilefraction and a viscosity of 1200 mPa.s in the solution described, asmeasured in an ICI plate-cone viscometer at 23° C.

PREPARATION OF COMPONENT (C) EXAMPLE 4

Blocked Polyisocyanate 1 (Component (C))

504.0 g of a commercially available isocyanurate trimer of hexamethylenediisocyanate and 257.2 g of the aromatic solvent described in Example 1are weighed out and charged to the apparatus described in Example 1equipped with a metering vessel and a reflux condenser. The solution isheated to 50° C. A mixture of 348.0 g of diethyl malonate, 104.0 g ofethyl acetoacetate and 2.5 g of a 50% solution of sodiump-dodecylphenolate in xylene is then metered from the metering vesselinto the solution over a period of 2 hours, such that the temperaturedoes not exceed 70° C.

The mixture is then heated slowly to 90° C., and this temperature ismaintained for 6 hours. A further 2.5 g of sodium p-dodecylphenolatesolution are then added and the mixture held at 90° C. until the contentof NCO groups in the reaction mixture has reached 0.48%. 35.1 g ofn-butanol are then added. The resulting solution has a nonvolatilefraction of 59.6% (measured in a circulating-air oven; 60 min at 130°C.) and a viscosity of 590 mPa.s, as measured in an ICI plate-coneviscometer at 23° C.

EXAMPLE 5

Blocked Polyisocyanate 2 (Component (C))

The procedure is as in Example 4. 722.0 g of a commercially availableisocyanurate trimer of hexamethylene diisocyanate and 460.0 g of thearomatic solvent described in Example 1 are weighed out and charged tothe apparatus. A mixture of 527.0 g of diethyl malonate, 130.4 g ofethyl acetoacetate and 4.5 g of a 50% solution of sodiump-dodecylphenolate in xylene is weighed out and transferred to themetering vessel, and metered in and reacted as described. A temperatureof 90° C. is maintained until the content of NCO groups in the reactionmixture has reached 0.92%. 20.3 g of 1,4-dimethylolcyclonexane are thenadded, and the mixture continues to be held at 90° C. until the contentof NCO groups in the reaction mixture has reached 0.28%. The batch isthen cooled and 140 g of n-butanol are added. The resulting solution hasa nonvolatile fraction of 56.8% (measured in a circulating-air oven; 60min at 130° C.) and a viscosity of 405 mPa.s, as measured in an ICIplate-cone viscometer at 23° C.

PRODUCTION OF TRANSPARENT TOPCOATS ACCORDING TO THE INVENTION

The transparent topcoats are produced by weighing out components (A),(B) and (C) in the order given in Table 1 and mixing them thoroughly bystirring with a laboratory turbine mixer, then adding butyl glycol orthe initial quantity of xylene and likewise stirring it in thoroughly.The UV absorber and the free-radical scavenger are premixed separatelywith (the second quantity of) xylene until they are completelydissolved, then they are added to the first portion of the formulationand likewise stirred in thoroughly. n-Butanol and the leveling agent arethen added and mixed in thoroughly. If appropriate, the resultingcoatings are adjusted for application with xylene to a viscosity of 23sec, measured in the DIN 4 cup at 20° C.

                  TABLE 1                                                         ______________________________________                                        Example           6         7      8                                          ______________________________________                                        Acrylate resin 1  --        53.8   --                                         Acrylate resin 2  72.7      --     --                                         Alkyd resin as in Example 3                                                                     --        --     66.7                                       Melamine resin.sup.1                                                                            14.1      11.5   14.1                                       Blocked polyisocyanate 1                                                                        6.8       --     --                                         Blocked polyisocyanate 2                                                                        --        13.2   6.8                                        Butylglycol       4.7       --     2.7                                        Xylene            --        8.0    --                                         UV absorber.sup.2 1.1       1.7    1.1                                        Free-radical scavenger.sup.3                                                                    1.1       1.5    1.1                                        Xylene            3.0       5.3    3.5                                        n-Butanol         4.0       5.0    2.0                                        Leveling agent.sup.4                                                                            2.0       2.0    2.0                                                        Weighed amounts in g                                          ______________________________________                                         .sup.1 Commercial melamine resin, highly etherified with nbutanol, with       residual imino [sic] groups                                                   .sup.2 Commercial UV absorber based on a substituted benzotriazole            .sup.3 Commercial freeradical scavenger based on a 1,4,6substituted           piperidine                                                                    .sup.4 5% solution of a polythethersubstituted polydimethylsiloxane in        xylene                                                                   

PRODUCTION OF TWO-COAT FINISHES OF THE BASECOAT/CLEARCOAT TYPE EXAMPLE 9

Steel panels coated with a commercially available, cationicallydeposited electrodeposition coating and a commercially availablesolvent-containing filler based on polyester and melamine resin aresprayed with a commercially available aqueous basecoat as described inEP-A-089,497 in two passes with a flash-off period of 1 minute inbetween so as to give a dry film thickness of 14 μm. The panels are thengiven a flash-off treatment for 10 minutes at room temperature and 5minutes at 80° C. and then the coating of Example 6 according to theinvention is applied in two spray passes so as to give a dry filmthickness of 43 μm. The panel is then given a flash-off treatment for 5minutes at room temperature and then baked for 20 minutes at 140° C.

EXAMPLE 10

The procedure is as in Example 9. After the application of the basecoatthe coating of Example 7 according to the invention is applied in twospray passes so as to give a dry film thickness of 45 μm. The panel isgiven a flash-off treatment for 5 minutes at room temperature and thenbaked for 20 minutes at 140° C.

EXAMPLE 11

The procedure is as in Example 9. After the application of the basecoatthe coating of Example 8 according to the invention is applied in twospray passes so as to give a dry film thickness of 44 μm. The panel isgiven a flash-off treatment for 5 minutes at room temperature and thenbaked for 20 minutes at 140° C.

TESTING OF THE TWO-COAT FINISHES

The test results can be taken from Table 2.

                  TABLE 2                                                         ______________________________________                                        Finish of Example 9         10     11                                         ______________________________________                                        Hardness                                                                      Fisher-scope (N/mm.sup.2)                                                                       --        142    --                                         Pendulum attenuation (Konig)                                                                    110       --     113                                        Gloss (Gardner, 20°)                                                                     88        85     87                                         Acid resistance                                                               2.sup.h 36% sulfuric acid*                                                                      --        0      --                                         4.sup.h 36% sulfuric acid*                                                                      --        1      --                                         6.sup.h 36% sulfuric acid*                                                                      --        1      --                                         Gradient oven, 1% sulfuric                                                                      44/51 ? 0 --     44/47 ? 1                                  acid**                                                                        Cross-hatching (cross hatch 1)*                                                                 0         0-1    0                                          Color comparison after                                                                          0-1       0-1    0                                          constant climatic test (240.sup.h)*                                           Yellowing (30 min, 160° C.)*                                                             0         0      0-1                                        ______________________________________                                         *Grading: 0 to 5, 0 = no damage, 5 = heavy damage                             **Test according to LPV 5500 of Mercedes Benz Werke                      

We claim:
 1. Process for the production of a two-coat finish, comprisingthe steps of:(1) applying a pigmented basecoat to a substrate surface,(2) forming a polymer film from the basecoat applied in step (1), (3)applying a nonaqueous transparent topcoat to the resulting basecoatlayer, which topcoat comprises(A) resins selected from the groupconsisting of synthetic resins containing hydroxyl groups and mixturesthereof, (B) resins selected from the group consisting of amino resinsand mixtures thereof, and (C) compounds selected from the groupconsisting of blocked polyisocyanates and mixtures thereof, wherein theisocyanate(s) is blocked with a blocking agent I, selected from thegroup consisting of dialkyl malonates and mixtures thereof and ablocking agent II, which is different from I and is selected from thegroup consisting of compounds containing active methylene groups, anoxime and mixtures thereof and the ratio of equivalents between theisocyanate groups blocked with (I) and the isocyanate groups blockedwith (II) being between 1.0:1.0 and 9.0:1.0, and subsequently (4) bakingtogether the basecoat layer and topcoat layer.
 2. Process according toclaim 1, characterized in that the ratio of equivalents between theisocyanate groups blocked with (I) and those blocked with (II) isbetween 8.0:2.0 and 6.0:4.0.
 3. Process according to claim 1,characterized in that component (A) has a hydroxyl number of from 40 to240, and is present in an amount of from 50 to 90% by weight, andcomponent (B) is present in an amount of from 5 to 45% by weight, andcomponent (C) is present in an amount of from 5 to 45% by weight, thepercentages by weight being based on (A)+(B)+(C)=100% by weight. 4.Process according to claim 1, characterized in that the blockedisocyanate groups contained in component (C) are attached to aliphaticor cycloaliphatic radicals.
 5. Process according to claim 1,characterized in that component (C) comprises an isocyanate selectedfrom the group consisting of blocked hexamethylene diisocyanate, blockedisophorone diisocyanate and mixtures thereof.
 6. Process according toclaim 1, characterized in that the blocking agent (I) is diethylmalonate.
 7. Process according to claim 1, characterized in that theblocking agent (II) contains active methylene groups is selected fromthe group consisting of alkyl acetoacetates having 1 to 6 carbon atomsin the alkyl moiety and mixtures thereof.
 8. Process according to claim1, characterized in that the blocking agent (II) is methyl ethylketoxime.
 9. Transparent topcoat composition of a two-coat finishincluding a basecoat and a transparent topcoat composition comprising(A)a resin selected from the group consisting of synthetic resinscontaining hydroxyl groups and mixtures thereof, (B) a resin selectedfrom the group consisting of amino resins and mixtures thereof, and (C)a compound selected from the group consisting of blocked polyisocyanatesand mixtures thereofwherein the isocyanate(s) is blocked with a blockingagent I, selected from the group consisting of dialkyl malonates andmixtures thereof and a blocking agent II, which is different from I andis selected from the group consisting of compounds containing activemethylene groups, an oxime and mixtures thereof and the ratio ofequivalents between the isocyanate groups blocked with (I) and theisocyanate groups blocked with (II) being between 1.0:1.0 and 9.0:1.0.10. Composition according to claim 9, characterized in that the ratio ofequivalents between the isocyanate groups blocked with (I) and thoseblocked with (II) is between 7.5:2.5 and 6.5:3.5.
 11. Compositionaccording to claim 9, characterized in that component (A) has a hydroxylnumber of from 40 to 240, and is present in an amount of from 50 to 90%by weight, and component (B) is present in an amount of from 5 to 45% byweight, and component (C) is present in an amount of from 5 to 45% byweight, the percentages by weight being based on (A)+(B)+(C)=100% byweight.
 12. Composition according to claim 9, characterized in that theblocked isocyanate groups contained in component (C) are attached toaliphatic or cycloaliphatic radicals.
 13. Composition according to claim9, characterized in that component (C) comprises an isocyanate selectedfrom the group consisting of blocked hexamethylene diisocyanate, blockedisophorone diisocyanate and mixtures thereof.
 14. Composition accordingto claim 9, characterized in that the blocking agent (I) is diethylmalonate.
 15. Composition composition according to claim 9,characterized in that the blocking agent (II) contains active methylenegroups and is selected from the group consisting of alkyl acetoacetateshaving 1 to 6 carbon atoms in the alkyl moiety and mixtures thereof. 16.Composition according to claim 9, characterized in that the blockingagent (II) is methyl ethyl ketoxime.
 17. Composition according to claim9, characterized in that the ratio of equivalents between the isocyanategroups blocked with (I) and those blocked with (II) is between 8.0:2.0and 6.0:4.0.
 18. Composition according to claim 9, characterized in thatcomponent (A) has a hydroxyl number of from 60 to 150 and is present inan amount of from 60 to 75% by weight, and component (B) is present inan amount of from 10 to 25% by weight, and component (C) is present inan amount of from preferably 10 to 25% by weight, the percentages byweight being based on (A)+(B)+(C)=100% by weight.
 19. Compositionaccording to claim 9, characterized in that the blocking agent (II) isethyl acetoacetate.